They're a pretty big contributor to Linux, so even if you don't see their work you're probably using it anyway. https://lwn.net/Articles/915435/

added value that cannot be fulfilled by independent experts or FOSS community

Wrong question IMO. It's more relevant to ask "without Red Hat, would independent experts or the FOSS community have added the same value?". Sure, it's possible that Red Hat has some highly skilled developers that possess unique skills required for their contributions, but in general contributing to FOSS projects is more about willingness to spend large amounts of time and resources on something that doesn't give you money in return.

Lots of large companies "could" have spent thousands of hours contributing to Linux, but unless they actually do it then it is irrelevant.

I worked for a bank. When they decided to deploy Linux on their infrastructure, they chose RHEL and they have signed a big contract with RedHat for tech support.

Overall, they chose RedHat for the same reason they chose Microsoft before: tech support. They have >10000 engineers, and yet somehow they think they absolutely need tech support... They pay a lot for it. In my building, they even got a Microsoft engineer once a week on-site until Covid. I don't know for the other people working for this bank, but I asked for Microsoft support only once in 2 years. In the end, their guy sent me back an email telling me "I've transmitted your question to the corresponding engineering team" and .. diddlysquat.

Now to be fair, for paying customers, RHEL and Microsoft both ensure security updates for a really a long time. Red Hat pays a lot of people to backport security patches from upstream to previous versions. It allows companies like the bank I worked for to keep running completely crappy and obsolete software for an insane amount of time without having to worry too much about security vulnerabilities.

Anyway regarding RedHat contributions, a lot of them are subtle.

• A friend of mine works for RedHat. He is a core Python developer and is paid full-time by RedHat to work on Python.
• Through this friend, I applied for a position in their company at some point (unfortunately, it didn't happen ; don't remember why exactly). The position was in a team dedicated to improve hardware support. They have built an infrastructure to let computer manufacturers (Dell, Lenovo, etc) test the compatibility of their new hardware with Linux/RHEL quickly and automatically.
• Part of the technical support they provide to some clients is "making things work". It may imply fixing bugs or improving drivers and then sending patches upstream.
• If I'm not mistaken, they paid Lennart Poettering to work on Systemd and Pulseaudio.
• They pay for the development of some infrastructure software like Corosync for instance.

This list is far from exhaustive. I'm sure they have paid for a lot of other things you're using daily without knowing it.